The present invention relates to a method of recording characters, an image, binary information or the like in high density on a semiconductor base plate on the basis of a novel recording principle.
Magnetic recording and read-out systems using magnetic (recording) tape is widely used at present as systems for semipermanently recording and storing various pieces of information and for reading them out when necessary. Nevertheless, since there is a limit in the fabrication accuracy of a magnetic head for such a system, the recording density of the magnetic recording system has a pitch of 1 .mu.m at best so that a recording density at an interval of sub-microns cannot be expected. As a result, the magnetic system cannot be compact in both bulk and weight in view of the storage of a recording medium, and this raises a large obstacle in a recent information society.
If the recording density is at a pitch of 0.1 .mu.m, the recording medium has its area and bulk reduced to one hundredth and one thousandth, respectively, relative to those in the case of 1 .mu.m pitch so that the storage apparently becomes remarkably advantageous. Therefore, there have heretofore been made several proposals which are intended to perform the records at the 0.1 .mu.m pitch.
From the macroscopic point of view, on the other hand, the recording operation is the same as as the writing operation of letters on a sheet of paper by means of a pencil, which requires the recording pencil and the suitable paper. In order to reduce the size of the letters written, moreover, there would apparently be required a pencil having a thin lead, paper of fine texture and an enlarger for observation.
From that analysis, there have heretofore been made a number of proposals in which a thin-focused charged particle beam (such as an electron beam or an ion beam) is used as the pencil having a thin lead.
Here, the principle of the prior art will be described merely as an example with reference to FIGS. 1A and 1B. As the recording medium, there is used a semiconductor junction (e.g., a p-n junction) which is constructed of an n-type semiconductor layer 2 and a p-type semiconductor layer 3, for example. Now, if the n-type semiconductor layer 2 is irradiated with an ion beam 1 which is accelerated to about 60 kV, for example, a lattice defect is generated in the irradiated area 4 because of the so-called "radiation damage" (as shown in FIG. 1A). If, at this state, the n-type semiconductor layer 2 is irradiated with an electron beam 5 which is accelerated to about 10 kV, carriers are generated in the n-type semiconductor layer 2, as is well known in the art (as shown in FIG. 1B). It is also known that a voltage is generated at the semiconductor junction because of the generation of those carriers. As a result, if two electrodes 6 and 6' are attached to the semiconductor layers 2 and 3, respectively, and are connected with a voltmeter 7, the generation of the voltage can be observed. Here, if the area 4 damaged by the irradiation is further irradiated with the electron beam 5, the generation of the carriers is obstructed by the radiation damage so that the indication of the voltmeter 7 is abruptly decreased. The area 4 written by the ion beam 1 can be detected by means of the electron beam 5. Since the ion beam 1 can be focused to have its diameter equal to or smaller than 1 .mu.m, the recording pitch can also be reduced to match the diameter of the ion beam 1. Thus, it is apparent that the rise in the recording density can be expected.
However, the micro-recording method thus far described still has several defects over and above the difficulty in generating such an ion beam as has a high current density and a small diameter. More specifically, since the semiconductor junction is used as the recording medium, (1) the diffusion process for the p-n junction is required. Moreover, since the electrodes 6 and 6' are required for detecting a weak signal corresponding to the weakly recorded area 4, (2) the formation process for the electrodes is required. And, if the p-n junction is prepared by thermal diffusion, impurities diffuse in both the sides of the semiconductor wafer providing the base plate, as is well known in the art, so that the p-n junctions are formed in the two sides. As a result, since removal of at least one of the p-n junctions becomes necessary, the number of processes is further increased. On the contrary, if the p-n junction is prepared by the ion implantation process, an annealing process is added for the activation of the p-n junction, as is well known in the art. In either case, the number of the processes inclusive is increased. Still moreover, if the reuse of the recording medium is, required (3) both the steps of removing the electrodes and the junction are required.
FIGS. 2A to 2D show another recording example according to the prior art using an electron resist and an electron beam. Although there are many kinds of the electron resists, a PMMA resist of an acryl resin is suitable for the micro-recording operation. Usually, an optically opaque and electrically conductive metal film 9 is attached to a glass plate 8. An electron resist film 10, having a thickness of about 3000 .ANG., is coated upon the metal film 9 (as shown in FIG. 2A). A Cr film having a thickness of about 700 .ANG. is frequently used as the metal film 9. If the resist film 10 is then irradiated with the electron beam 5 (as shown in FIG. 2B), the chemical bonds are broken in the case of the PMMA resist, for example, and the irradiated area 11 is removed during the developing process (as shown in FIG. 2C). Then, that resist film 10 is used as a mask to etch the metal film 9 and is finally removed. As shown in FIG. 2D, a micro-recorded area 11' is fixed as a hole in the metal film 9. As a result, the recorded area 11' can be read out by enlarging it with an optical microscope and by projecting it if it constitutes a letter, for example.
However, the method thus far described has so many fabrication processes that it is apparently unsuitable for processing much information. Moreover, the method shown in FIGS. 1A and 1B can effect the read-out immediately after the recording process using the ion beam, whereas the method shown in FIGS. 2A to 2D cannot effect the read-out so far as at least the developing process is ended.